We frequently have questions that are crossposted, e.g. https://mathoverflow.net/questions/137414/systems-of-parameters-for-algebra-a#question.

The only options for now are to close as off-topic, leaving a comment pointing to the math.SE question, or to migrate and leave math.SE to clean up the mess. Neither is that great, and the perfect solution would be to close as a duplicate of the math.SE question. However, if you paste the math.SE URL into the "close as duplicate" dialog, the software responds with "The duplicate question must exist on MathOverflow"

Can we fix this, and allow closing as a duplicate of a math.SE question?

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    $\begingroup$ An additional issue regarding cross-posts (in particular when really made in parallel) could be that AFAIK now one can only close as duplicate of something that has an answer (this is for normal users, you, as a moderator, still can, I think). So, to make this really effective one would need perhaps an exception to this, too. $\endgroup$
    – user9072
    Commented Jul 25, 2013 at 2:26
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    $\begingroup$ Related feature request on meta.SO (currently incorrectly closed as duplicate): meta.stackexchange.com/questions/94322/close-as-cross-post $\endgroup$ Commented Jul 25, 2013 at 4:00
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    $\begingroup$ Related meta.MSE thread: meta.math.stackexchange.com/questions/10265/… $\endgroup$
    – Asaf Karagila Mod
    Commented Jul 25, 2013 at 8:07
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    $\begingroup$ The only options for now are to close as off-topic, leaving a comment, or to migrate. Not sure if it is any better than these solutions, but users having sufficient reputation can vote to delete a question. And, of course, moderators can delete questions. $\endgroup$ Commented Jul 26, 2013 at 5:27
  • $\begingroup$ Here I posted a question that I had previously posted to math.stackexchange.com, and here is another, and here is another. Are there thoughts on the propriety of posting these to both forums rather than only one of them? $\endgroup$ Commented Jul 26, 2013 at 20:24
  • $\begingroup$ AFAIK the rule of thumb to follow is to wait for a while for answers on the one site and only then post to the other (still mentioning one posted it elsewhere before). It seems you did so, thus in my opinion this is fine. $\endgroup$
    – user9072
    Commented Jul 26, 2013 at 20:47
  • $\begingroup$ @quid: Do you mean migrate it to the other site, or actually create a separate post? $\endgroup$ Commented Aug 9, 2013 at 13:59
  • $\begingroup$ @MichaelAlbanese I was talking about actually creating a separate post. Of coourse for requests for migration it also seems wise to wait a while before asking it to be migrate, yet in that case there seems no need, but one could if one wants to, to mention that it was on the other site before as this is visibly automatically. $\endgroup$
    – user9072
    Commented Aug 9, 2013 at 15:09
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    $\begingroup$ A related discussion: Close cross posts between here and MSE. $\endgroup$ Commented Sep 18, 2013 at 13:04

1 Answer 1


There is no such thing as a cross-site duplicate in the Stack Exchange concept. There are several sites with a significant overlap in their scopes, and some amount of duplication is unavoidable. In general it is not considered a valid close reason if a question is already answered on a different SE site. Multiple version of the same question are even quite useful in some cases as different sites have different audiences and the answers can look at the same question from quite different perspectives.

Having a cross-site duplicate close reason would likely lead to confusion about the rules as it would suggest that they are generally against the rules.

The problematic part are not cross-site duplicates, but users cross-posting immediately without regard for the waste of time this can cause to the answerers. While I previously said that duplicates are not necessarily bad, such simultaneous cross-posts can cause users to waste time on answers that were already posted on the other site.

I generally close cross-posts when they are identical and without any effort to adapt them to different sites. If the question fits better to one of the sites, I'll close or flag the question on the other sites. If it fits equally well to all sites I tend to close the version on the site I'm a moderator on, as that is easier than flagging.

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    $\begingroup$ The point of my feature-request was to ask that there be such a thing... :-) Perhaps the real feature-request should be for rapid fire (<n days?) cross-posting to be automatically noticed and forbidden by the software. $\endgroup$ Commented Jul 25, 2013 at 13:10
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    $\begingroup$ @ScottMorrison My main point is that the existence of such a feature would strongly suggest that cross-site duplicates are generally disallowed. $\endgroup$
    – user35354
    Commented Jul 25, 2013 at 19:39
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    $\begingroup$ I think we'd essentially like to disallow crossposts with math.SE. If you've already posted on math.SE, you shouldn't post the same question without waiting at least a couple of days without an answer. $\endgroup$ Commented Jul 26, 2013 at 0:22

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